Where these prior art units are concerned, the guide bush serves on the one hand to guide the piston rod while on the other the piston rod bears on the inside wall of the cylinder through its guide piston. To avoid excessive surface pressure due to not absolutely accurate alignment between the bore of the guide bush and the cylindrical inner wall of the housing, the guide bush, i.e. its guiding length, should not be too long, particularly if the piston rod with the guide piston is retracted relatively far into the housing. On the other hand, the guiding length should not be too small when the piston rod is substantially or completely extended out of the housing, in other words until the guide piston bears against the guide bush. Another factor against having a long guide bush is once again, particularly if the piston-cylinder unit is constructed as a gas spring the fact that gas space is lost and yet it is the gas space which is important to achieving the flattest possible characteristic curve for the gas spring.